Various lubricating oil additives each show low solubility in a synthetic hydrocarbon compound (synthetic base oil) which is obtained by turning 1-decene (linear α-olefin having 10 carbon atoms) into an oligomer and which is referred to as a poly-α-olefin (PAO). In view of the foregoing the solubility is improved by adding a dicarboxylic acid diester (such as diisononyl adipate) at a content of 20 to 40 mass % with respect to the PAO upon use of the PAO. An oligodecene which is obtained by turning 1-decene into an oligomer by using a metallocene catalyst and which has a number average molecular weight of 500 to 200,000 has been disclosed as an example of the PAO (see, for example, Patent Document 1).
A lubricating oil composition has been conventionally prepared by adding any one of various lubricating oil additives to base oil. However, the dissolution of any one of those additives in the base oil requires an increase in polarity. When the oligodecene described in Patent Document 1 described above is used as base oil, and an additive is added to the oligodecene, the sufficient dissolution of the additive in the base oil requires the blending of expensive diisononyl adipate at a content of 30 mass % or more with respect to the oligodecene.
Patent Document 1: Japanese Patent Translation Publication No. 2002-518582